AN: This is for the Nonny who prompted me on Tumblr with "The tragic romance of aquarium fish in two separate tanks," for an obscure AU. That was certainly AU, and at first I didn't think I could do it. Then I remembered the old quote about "a bird may love a fish, but where would they live?" and I got the idea to make the fish a metaphor. Not quite what you asked for, but hopefully you like it!

Rose sighed as the Doctor paid their entrance fee to the aquarium. Ever since their narrow escape during the Battle of Canary Wharf, he'd taken them on one safe excursion after another. She knew he was running scared, but three weeks had passed, and she was beginning to think he was running from more than just the reminder that they'd almost lost each other.

She was tired, though. Tired of running, tired of putting on a happy face when she missed her mum so he wouldn't feel guilty. She didn't regret staying with him, not for a second, but if he knew she cried herself to sleep some nights, his guilt would be unbearable.

The Doctor grinned as he took her hand, and there was something frantic beneath the happiness that made her think he already knew. "Come on, Rose Tyler!" he chirped. "Best aquarium in two galaxies."

Rose nodded and smiled back at him, then let him drag her from one tank to another. But no matter how hard she tried, she couldn't summon up the energy to show enthusiasm for the creatures he showed her. His determination to pretend that all was well became panicked as the day went on and he failed to break her listlessness.

But finally an exhibit caught her eye. Two tanks, side by side, containing two specimens the label told her were of the same species. Both tanks were large, but neither fish took advantage of the space. Instead, they swam along the glass wall dividing them, stopping now and again to stare at each other.

"Doctor, look," Rose said, pointing at the blue and white striped fish.

The Doctor shoved his hands into his pockets and rocked back on his heels. "Ah. Mortiga fish." He didn't sound pleased that she'd noticed them.

"Why are they in separate tanks?" Rose asked. "You can tell they want to be together."

He looked down at her with a pale imitation of his "dribbled on your shirt" look. "Really, Rose?" He wrinkled his nose. "Writing love stories for our fishy friends now?"

Rose scowled up at him. "Mock me if you want, Doctor, but I know what I see," she said stubbornly. "They're staying as close to each other as they can get, and they keep staring at each other through that cruel glass that separates them. This is a mated pair, and the aquarium is keeping them apart for some reason."

The Doctor sighed and ran his hand through his hair. "Because she'll die if they don't," he said, his voice subdued. "Mortiga fish are rare for a reason. Mating and breeding kills the female. That's why they lay huge clutches of eggs at a time, but they leave their mate to hatch the eggs alone." He pressed his hand to the glass. "Aquariums figured out a long time ago that the only way to keep a pair was to put them in separate tanks."

Rose stared at the fish, her heart racing. "But… but what if it would be worth it to her?" she asked. "You can't tell me she's happy being kept away from her mate."

"She deserves to live," the Doctor said fiercely. "Oh, he would selfishly mate with her, if given the chance, but don't you see, Rose? This way, she can live. They might not be able to truly be together, but at least they can see each other and know they're both alive."

Rose knew they weren't talking about the fish any more—if they ever had been. The passion in his voice when he tacitly confessed that he wanted her sent a tendril of desire through her body, and she resolved that they would not leave this spot without her letting him know how much she wanted that, too.

"It isn't fair," she told him, keeping her gaze fixed on the fish so he wouldn't see the longing in her eyes. "Everyone else is making the decisions for her. Maybe she'd die, yeah, but doesn't she deserve to make that choice herself? Maybe she doesn't want to live a safe life, deprived of real happiness. Maybe she doesn't want to be in a parallel world with her mum and best mate when she could be here, with you."

"Rose."

The Doctor's voice broke on her name, and Rose finally turned to look at him. Tears shone in his eyes, and she could see his adam's apple working as he tried to control his emotions.

"I love you, Doctor," Rose finally confessed, grabbing his hand when he stumbled back in shock. "I love you, and I want to be with you—like a proper couple."

"I want that, too," he admitted hoarsely. "But you'll die one day and leave me alone, and I don't know…"

Rose shook her head. "Unless we're like these fish, and us making love would actually kill me, shouldn't we take the chance to be happy while we can?" She stepped closer and put a hand on his lapel. "Please, Doctor. I'm tired of staring at you through glass all the time. Aren't you tired of it, too?"

Silence hung between them for several heartbeats, and Rose had nearly given up when his hand landed on her hip and pulled her close. She closed her eyes when he pressed his forehead to hers and matched her breathing to his.

He sighed against her lips, and her eyes fluttered open to look at him. "No more glass, Rose Tyler," he whispered, then pressed his lips to hers.